This invention pertains to the art of packing arrangements and more particularly to an actuator stem packing arrangement. The invention is particularly applicable to a rotary valve that undergoes frequent cycling and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications and may be advantageously employed in other packing environments and applications.
Ball valves include a body having a chamber adapted to receive a ball member that is selectively rotated between open and closed positions. Upstream and downstream seat assemblies seal around a through flow passage provided in the ball member. An actuating stem extends from the ball member outwardly through the body to permit selective rotation of the ball member between valve open and closed positions. A stem passage is provided through the valve body to receive the actuating stem. Due to the high fluid pressures associated with these types of valves, particular emphasis is placed on the design of stem packing arrangements to seal between the actuating stem and valve body.
A number of stem packing arrangements have attempted to address this problem in an equally wide variety of manners. The most basic stem packing arrangement is an O-ring that provides a sliding type seal that accommodates rotary movement of the stem. More often, though, the stem packing assembly includes multiple elements that are placed under compression to tightly seal along the external surface of the actuating stem. By way of example, commonly assigned U.S. Pat. Nos. 4,538,790 to Williams, et al. issued Sept. 3, 1985 and 4,558,874 to Williams, et al. issued Dec. 17, 1985 illustrate two multiple element stem packing arrangements that have met with substantial commercial success.
In certain applications, valve cycling, i.e. opening and closing, becomes the primary criterion that must be addressed by the stem packing arrangement. Frequent valve cycling necessarily alters the service life of a valve due to the dynamic forces and effects resulting therefrom. The subject application is particularly applicable to cycling applications on the order of one cycle per second. Under such conditions, the design of the stem packing arrangement must be reexamined in order to accommodate the sealing and bearing function of the stem packing arrangement under such strenuous conditions.
Simultaneously, the stem packing arrangement must accommodate a high pressure environment which, in turn, dictates the design characteristics of the individual packing elements. This unique set of circumstances, that is frequent cycling and high pressure, is not adequately met by prior stem packing designs that would otherwise seem suitable for a particular set of circumstances or another. It has, therefore, been deemed desirable to provide a stem packing arrangement that satisfies these specialized design criterion.